The Area of Validity map on the SBB site shows dotted lines above Wengen indicating a reduction. There is a circled "1" indicating a 25% discount, but it's unclear if that applies to the entire route to Jungfraujoch from Wengen, or just above Kleine Scheidegg.

Given the above sources, a Swiss Travel pass could give discounts anywhere from 25% to 100% for the trip between Wengen and Kleine Scheidegg. Does anyone know what the actual discount is?

You can very safely use the Area of Validity page (www.myswissalps.com/sw isstravelpass/validity) as the authority on the subject. The ① refers to all the red-dotted lines above (i.e. higher up the hill than) Wengen/Grindelwald.